Direct-Reading Compass at 46 Degrees South
You are the pilot in command of a twin turboprop whose slaved heading system has failed in flight, with heading flags on the HSI and flight director. You are now flying on the standby attitude indicator, the turn coordinator and the direct-reading magnetic compass. You are in IMC at night, cleared to hold at the Invercargill VOR (45°S, magnetic dip approximately 64°) and then to carry out the VOR/DME approach. The compass deviation card is current and shows errors of 1-2° on the cardinal headings. The aircraft is clean at 180 kt, the air is smooth and the autopilot is disengaged. You plan standard-rate turns and will decelerate to 140 kt on the inbound leg before configuring. Your first turn after crossing the VOR takes you from 090°M onto the 180°M outbound, and the inbound course to the VOR is 270°M.
1. During the turn from 090°M onto 180°M, what will the compass indicate as the aircraft passes south, and how will you roll out accurately? Explain the cause.
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The compass will lag the turn, and you must roll out early.
Cause - dip. At 45°S the earth's field has a large vertical component pulling the north-seeking end of the magnet downwards (dip about 64°). The magnet is pendulously mounted, so in a bank the resultant of gravity and centrifugal force tilts the pivot axis and the card rotates about the vertical. The resulting error - turning error - is zero on east/west headings and maximum on north/south headings.
In the southern hemisphere the effect on a turn through south is to under-read. When the aircraft is actually on 180°M the compass may show roughly 165-172°M, depending on bank angle and rate. If you rolled out when the compass read 180, you would settle 10-15° past the outbound heading and then have to correct back.
Technique: begin the roll-out before the compass reaches 180 (at about 170°M), use the turn coordinator plus the VOR bearing and DME to judge the turn, then let the compass settle after the wings are level and check it against the VOR radial before starting timing.
2. Inbound on 270°M you reduce power and decelerate to 140 kt to configure. What will the compass do, and what would it have done in the northern hemisphere?
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On 270°M you are on an east-west heading, so turning error is negligible but acceleration error is at its maximum. On east or west headings the horizontal component of the field lies along the aircraft's axis, so a horizontal force on the pendulous magnet produces a false rotation about the vertical axis.
In the southern hemisphere the rule is the reverse of the northern 'ANDS':
- Decelerating (reducing power at 270°M) the compass indicates a turn towards north - the heading appears to swing right, towards 360.
- Accelerating it would indicate a turn towards south.
The deflection is typically 5-15° with moderate acceleration and is transient; it disappears when the speed stabilises. Do not correct for it. Hold wings level on the attitude indicator and accept that the compass can only be read in steady, unaccelerated flight.
In the northern hemisphere the same power reduction would produce a false turn towards south. Configuration and power changes on final are exactly the phase where crews are most often misled by this error.
3. State the operational limits of the direct-reading compass for this approach and the mitigations you will apply.
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The compass is only reliable in steady, unaccelerated flight with the wings level, and it oscillates in turbulence. Its accuracy also depends on the deviation card, which cannot compensate for dip errors. For this approach:
- Read the heading only after the aircraft is stabilised, and apply the deviation card.
- Use the VOR radial and DME as the primary track reference and fly published headings with the turn coordinator and timing rather than chasing the compass.
- Keep bank to rate 1 or less; roll out early through south and late through north.
- Cross-check wings-level on the standby attitude indicator and turn coordinator, not on the compass.
- Tell ATC you have lost the slaved heading reference. With one heading source, an IMC night approach to a VOR/DME with terrain nearby deserves the higher minima, or a diversion if turbulence makes turns imprecise.
- Report the failed heading system and any compass suspicion after flight - a serviceable compass swing cannot fix dip error, but a bent or leaking compass bowl can.